What's the Best Way to Spread the Word About a New Product?

The value of public relations is in the credibility it can create as a result of a third-party, such as a TV, newspaper or other media story, says your product or service has value. You'll need to think in terms of a newsworthy angle for your product's story.
-- Is there new research that shows your product can make consumers happier, healthier, thinner? Can you find some?
-- Is there a special project or event your company can sponsor or be part of?
-- Can it be tied to kids or a group in need?
Look for some case studies on how PR practitioners have done this for other products to use as models and guides. The classic case study is Edward Bernays's success in persuading the public that bacon and eggs was the ideal American breakfast. A modern case study would be Barack Obama’s PR efforts using social media to “sell” his presidential campaign.
Once you get the newsworthy angles down, you can see how you can leverage those "hits" into effective brand building strategies.
I agree with both Al Reis and Jack Trout that PR is the foundation of any kind of brand building. Consider reading Reis's book (co-authored with his daughter Laura) on the Fall of Advertising and Rise of PR.